In a mobile communication system using the CDMA method, transmission power control is carried out in order to solve the so-called near-far problem. Transmission power control is broadly divided into two kinds: open-loop and closed-loop. When closed-loop transmission power control is applied to downlinks (channels from a base station to mobile stations), the operation of mobile stations and the base station is as follows.
A mobile station measures the SIR (Signal to Interference Ratio) indicating the quality of a received signal, and compares this measured SIR with a target SIR. If the measured SIR is greater than the target SIR, the mobile station sends the base station an instruction to lower the transmission power, and if the measured SIR is less than or equal to the target SIR, the mobile station sends the base station an instruction to raise the transmission power. The base station increases or decreases transmission power in accordance with this instruction.
Thus, with closed-loop transmission power control performed on a downlink, a mobile station instructs the base station to raise or lower transmission power based on the amount of interference received by the mobile station.
With the CDMA method, a base station performs communication with a plurality of mobile stations at the same time in the same frequency band, and therefore an increase in transmission power for a particular mobile station causes an increase in the amount of interference for other stations. Also, a mobile station for which the amount of interference increases sends the base station an instruction to raise the transmission power in order to maintain reception quality. When the base station increases transmission power for the mobile station that transmitted the instruction, the amount of interference for other mobile stations increases.
When such a series of operations is repeated, transmission power for all mobile stations gradually increases, and the amount of interference for other mobile stations also gradually increases. When the amount of interference mutually imposed by mobile stations gradually increases in this way, it is possible that the mutually imposed amount of interference will increase to an impermissible extent. As a result, the number of mobile stations that a base station can accommodate in its cell will be drastically reduced. That is to say, system capacity will be drastically reduced.